Clearcoat materials for automotive finishing preferably possess good optical properties (appearance). In addition, however, it is also desirable that, for the purpose of improved environmental compatibility, the clearcoat materials have a low solvent content, i.e., a high solids fraction. Increasing the solids fraction of a clearcoat material usually entails, however, a deterioration in the optical properties, owing to the increase in the viscosity of the clearcoat material.
Generally speaking, one of the factors on which the viscosity of a polymer solution depends is the dimension of the polymer, i.e., the hydrodynamic radius of the dissolved molecule. The hydrodynamic radius of the polymer is influenced in particular by the solvation of the polymer chains by the solvent, which differs sharply according to the particular solvent. Therefore, in order to achieve a low viscosity in spite of a high solids fraction in a polymer solution, it is in principle possible to use solvents which bring about only a low level of solvation of the polymer chains (theta solvents). Disadvantageous consequences of doing so, however, include the nonuniversal usefulness of these solvents with all polymers, and also the high costs of these solvents, depending on the resins employed.
Another way of achieving a low solution viscosity and hence effective leveling at the same time as a high solids fraction is to use very compact macromolecules, such as, for example, to use star polymers and hyperbranched, dendritic compounds. It is known that compact macromolecules of this kind, in solution or in a composition, have a low viscosity in tandem with a comparatively high solids fraction (e.g., Roovers, J., Macromolecules 1994, 27, 5359-5364 and Roovers, J. et al., Macromolecules 1993, 26, 4324-4331). Using star polymers is suitable only for short polymer chain arms, since at high concentrations star polymers with long polymer chain arms are subject to repulsive interactions, which can lead to quasicrystalline ordering phenomena in solution. These ordering phenomena lead to an increase in the viscosity, which is a disadvantage for the reasons mentioned above. Hyperbranched, dendritic compounds which are not subject to such ordering phenomena are therefore generally preferred.
Hyperbranched, dendritic compounds, i.e., hyperbranched, dendritic macromolecules and dendrimers, can be described in general terms as three-dimensional, highly branched molecules having a treelike structure. Dendrimers are highly symmetrical, whereas similar macromolecules, referred to as hyperbranched and/or dendritic, may to a certain extent be asymmetrical and nevertheless retain the highly branched treelike structure. In general it is possible to prepare dendrimers having a narrow molar mass distribution; in other words, in that case they are monodisperse or substantially monodisperse hyperbranched macromolecules. For monodisperse compounds the ratio of the weight-average to the number-average molecular weight (Mw/Mn)=1, whereas for substantially monodisperse compounds Mw/Mn˜1. Hyperbranched and dendritic macromolecules can typically be prepared starting from an initiator or nucleus having one or more reactive sites and a number of branching layers (“generations”) and, if desired, a layer of chain-terminating molecules (divergent synthesis approach). The continued replication of branching layers normally produces an increased multiplicity of branching and, if appropriate or desired, an increased number of end groups. The layers are typically called generations and the branches dendrons.
Binders based on hyperbranched polyesters are prior art. For example, U.S. Pat. No. 6,569,956 B1 describes a hyperbranched polyester-polyol macromolecule having a multiplicity of external and internal hydroxyl groups located on it, it being possible to use these groups to produce coating compositions having a high solids fraction. The hyperbranched macromolecules described therein, however, have only a low level of hydroxyl functionalization, which is a disadvantage for the production of hard and chemical-resistant coating compositions.
WO 03/093343 A1 as well describes high-functionality, hyperbranched, hydroxyl functional polyesters which can be used in coatings and paints. The high-functionality hyperbranched polyesters it describes, however, are molecularly and structurally heterogeneous, meaning that, although they can be prepared with little effort, they can be used only conditionally to achieve a high solids fraction while at the same time imparting good optical properties to coating compositions. Moreover, the polyesters described therein lack general compatibility with nonpolar aprotic solvents.
WO 2004/020503 A1 describes a process for preparing hyperbranched, water-soluble or water-dispersible polyesters from dicarboxylic acids and polyetherpolyols which have at least 3 OH groups, these polyesters being suitable for use in paints and coatings. The polyesters it describes, however, are likewise molecularly heterogeneous polymers, which again can be used only with limitations for increasing the solids fraction while at the same time imparting good optical properties to coating compositions. Moreover, these polyesters too, lack general solvent compatibility.
EP 991 690 B1 describes a process for synthesizing polymeric polyalcohols composed substantially of polyester units, the process providing hyperbranched, dendritic polyesters having unprotected or protected hydroxyl end groups. The products of the process can be functionalized and/or terminated with various groups. No end use is stated for the polyesters preparable by the process described.
WO 93/17060 A1 discloses a hyperbranched, dendritic macromolecule composed substantially of polyester units. The composition of the macromolecule includes an initiator which has at least one hydroxyl group and to which at least one branching generation is added, this branching generation comprising at least one chain extender with at least one carboxyl group and at least two hydroxyl groups. If desired, the macromolecule is chain-terminated. The hyperbranched, dendritic macromolecules described are available inexpensively by way of the process which is likewise described. The macromolecules as described are suitable for uses including that as binders for systems with radiation curing. Their use in thermosetting clearcoat compositions, however, is not described. Furthermore, the hyperbranched, dendritic macromolecules set out in WO 93/17060 A1 are not suitable for use in clearcoat compositions featuring a high solids content and good optical properties.